Christchurch police say they have had strong words with the owners of a dairy that allegedly sold synthetic cannabis to patients from a nearby mental health hospital.

The Canterbury District Health Board confirmed that Hillmorton Hospital management complained to police about a dairy selling patients K2, an R18 drug that has a reputation for making users feel aggressive, depressive and even suicidal after the initial high.

K2 is also sold in video stores and adult stores and costs $20 a packet.

CDHB specialist mental health services manager Toni Gutschlag said hospital staff talked to police as they were concerned the dairy was selling K2 "on tick", where the patients could run a tab and pay it off at the end of the week.

The owners of the dairy said they only sold the drug to patients because they were with hospital staff - a claim the board said was "highly unlikely".

Managers at two other dairies in the area said they did not sell synthetic cannabis.

Gutschlag said staff would often accompany patients to the shops to buy personal items such as deodorant, but it was "highly unlikely that staff would stand by while mental health patients purchased K2".

"We actively discourage patients using K2 as we know first-hand of the harm it can cause. Inpatients are not able to use or store K2 while they were on the ward, so I would be very surprised if any CDHB mental health staff member would stand by and knowingly allow a patient to purchase this substance."

She said staff had reported problems with K2 had reduced in the past four to six weeks.

"Despite this, we are very concerned about the impact K2 can have, especially on people engaged in our services. People assume it is safe because you can buy it at the dairy, but it can have some serious adverse effects."

Police community safety team Sergeant Bevan Seal, part of a team that visits dairies across the city asking them to stop selling synthetic cannabis, has talked to people at the dairy in question.

Dairies not selling the products are given a poster to place in the front window, which police hope will encourage the public to support only those dairies.

Seal said the effects of K2 were far-reaching. He had met a teenage girl whose mother was so addicted to K2 that she smoked three to four packets a day, and was unable to buy food for her children. Police had also found a 13-year-old K2 user disoriented and walking in the middle of a busy road.

Christchurch Hospital emergency specialist Paul Gee said the side-effects of synthetic cannabis included agitation, confusion, paranoia, seizures and violent behaviour that could last days to months.